The invention relates to miscible polymer blends comprising polyvinylidene fluoride (xe2x80x9cPVDFxe2x80x9d) and its copolymers, and polyalkyl methacrylates having a molecular weight from about 25,000 grams per mole to about 200,000 grams per mole, including polymethyl methacrylate (xe2x80x9cPMMAxe2x80x9d). The terms polyvinylidene fluoride and polyalkyl methacrylate as used herein are meant to encompass polymers comprising these materials, including homopolymers of these materials and copolymers comprising these materials. Miscible polymer blends can be generally described as polymer blends that are homogeneous down to the molecular level associated with a negative value of the free energy of mixing. The invention provides miscible polymer blends with superior and unexpected physical characteristics, including solvent resistance, gloss retention and adhesion after weathering.
Polyvinylidene fluoride, in particular, PVDF is a very important raw material for architectural coatings based largely on ultraviolet (xe2x80x9cUVxe2x80x9d) light transparency. PVDF, for example, provides long term protection for metal surfaces against exterior exposure, much better than other known coating materials. In order to balance the performance of polyvinylidene fluoride coatings, such as PVDF coatings, a secondary resin normally is needed to provide good adhesion to substrate, to reduce the shrinkage of the polyvinylidene fluoride due to excess crystallization, to increase the dispersability of a pigment in a coating and to obtain good optical properties. The selection of the secondary resin is based largely on the miscibility of the resin with polyvinylidene fluoride polymers.
Blending polymers has been an important industrial approach towards development of polymeric coating materials, however much of the development work has been based on compatibility of polymers. PVDF is compatible with a few industrial polymers, including polymers of alkyl methacrylate and alkyl acrylates with carbon content on alkyl groups of less than 3. The polymers miscible with PVDF, as well as other polymers comprising polyvinylidene fluoride, have the common characteristic of high concentrations of Cxe2x95x90O groups, particularly on side chains of the carbon polymer.
PVDF is thermodynamically miscible with polymers of alkyl methacrylate and it is discovered by the inventors that PVDF is thermodynamically miscible with polymethyl methacrylate having a molecular weight of from about 25,000 grams per mole to about 250,000 grams per mole which, as a miscible polymer blend, unexpectedly results in a blend having superior physical characteristics favorable for weather resistant coating materials.
U.S. Pat. Nos. 4,770,939 and 5,030,394 to Sietses et al. (xe2x80x9cSietsesxe2x80x9d) describe a process for preparing blends comprising PVDF having molecular weight of between about 10,000 and 70,000 with thermoplastic PMMA. According to Sietses, coating materials made with the process of that invention with PVDF having molecular weights above 70,000 are said to be unacceptable because the coating material remains too viscous when treated with heat during application on a substrate. The blends in Sietses are cryogenically ground to a powder and require the use of a flow promoter. Powder coating requires a low melt viscosity. Therefore, high molecular weight polymers are not suitable for such application. This is different from dispersion coating such as that of the invention, which allows the use of polymers having from low to high molecular weight. The Sietses patent concerns the use of low molecular weight polymers for processibility rather than the optimization of coating performance.
For comparison, Sietses prepared a dispersion coating comprising PVDF with a molecular weight of about 350,000 and viscosity of about 31,000 poise and an unspecified thermoplastic methyl methacrylate polymer. Because of the lack of optimization it has only 85% gloss retention after 2,000 hours. The current invention exhibited the optimized polymethyl methacrylate structure, that is a molecular weight of between about 25,000 grams per mole and about 200,000 grams per mole to provide 100% of gloss retention after 5,000 hours of QUV exposure as illustrated in FIG. 2.
U.S. Pat. No. 3,324,069 to Koblitz et al. discusses dispersions of PVDF and acrylate polymer. The disclosure of this patent does not address miscibility of the polymer and characterizes the compositions as alloys. This patent indicates that addition of more than 25% acrylate polymer is unfavorable as it causes a significant reduction in the properties of the PVDF.
U.S. Pat. No. 5,082,616 to White (xe2x80x9cWhitexe2x80x9d) describes a method for preparing biaxially oriented PVDF/PMMA films comprising blends of PVDF with a molecular weight of about 330,000 to about 390,000 and PMMA with a molecular weight of about 110,000 to about 125,000. The amount PMMA in the film, or other polymers, is from about 10% to about 30% of the film, and it is said in White that addition of more than about 30% other polymer detrimentally affects the physical properties of the film. The biaxially oriented film is made by a double bubble process.
The properties and characteristics of PVDF and PMMA blends can be fashioned to achieve purposes of a particular application. In White, for example, PVDF/PMMA blends are formulated to obtain films and not coating compositions. An aspect of the invention in White is retarding the crystallization of PVDF during the double bubble processing method used to obtain the film.
Thus, the film obtained by the method in White comprises PVDF with a lower crystallinity than the PVDF in blends of the invention. With respect to the coating compositions of the invention described herein, it is desired to maintain the crystalline structure of the vinylidene fluoride as this results, in part, in the unexpected properties of the weather resistance and solvent resistance of the coating compositions. The coating composition maintains crystallinity of the vinylidene fluoride polymer by employing a solvent preparation method. The polymers are blended in a latent solvent and then a pigment is added. The solvated blend is then heated to remove the solvent thus forming the coating composition. The coating composition of the invention has highly crystalline polyvinylidene fluoride and the retardation of crystallization of the polymer during coating formation.
The invention described herein is a coating used on substrates. White describes PVDF films capable of exhibiting desirable piezoelectric properties and, thus, White does not describe weather coating compositions, and characteristics such as gloss retention and solvent resistance are not important or characteristics of the piezoelectric films of White. It is not uncommon for polymers with similar content to be different and exhibit different characteristics based on internal or molecular structural differences. White, for example, concerns a biaxially oriented film with PVDF having retarded crystallinization as contrasted with the coating preparation of the invention. The invention exhibits superior qualities such as gloss retention and solvent resistance after QUV exposure. This is achieved, in part, by the crystalline structure of the polyvinylidene fluoride, such as PVDF, which remains significantly unaffected during blending of the vinylidene fluoride polymer with other polymers.
U.S. Pat. No. 3,607,754 to Asahina concerns the use of PVDF/PMMA blends for their piezoelectric properties and concerns blends with very high molecular weight PMMA, that is about 1,000,000 grams per mole, for use as electrets. U.S. Pat. No. 4,822,122 to Yamamato concerns PVDF/PMMA blends used as optical transmission media. These patents provide further examples of the use of vinylidene fluoride polymer and polyalkyl methacrylate blends formulated to achieve desired properties and characteristics for particular applications.
The present invention utilizes miscible blends comprising polyalkyl methacrylates such as PMMA, within a certain molecular weight range and vinylidene fluoride polymers, such as PVDF for coatings which, surprisingly, affects the physical characteristics of the coating, including solvent resistance and gloss retention. Since polyvinylidene fluoride particles undergo plasticization and mixing with solvent and polyalkyl methacrylate, respectively, the development of physical properties during the formation of the blend is a kinetic process. Initially, the solvent swells and solvates vinylidene fluoride polymer particles, and eventually the solvated vinylidene fluoride polymer is mixed with the polyalkyl methacrylate. It has been found that the uniformity of the mixing depends on the molecular weight of the polyalkyl methacrylate.
It is discovered that a miscible blend of polymers comprising polyvinylidene fluoride and polymers comprising polyalkyl methacrylates within a certain molecular weight range results in a coating composition with superior physical properties including hardness, gloss retention and solvent resistance. The polymers comprising polyvinylidene fluoride may be a homopolymer of vinylidene fluoride, that is PVDF, or a copolymer of more than about 80% vinylidene fluoride and up to about 20% hexafluoropropylene. The polyalkyl methacrylate may be the homopolyer of methyl methacrylate, that is PMMA, but can also be a copolymer comprising at least 65% methyl methacrylate and up to 35% other alkyl methacrylates such as ethyl methacrylate and butyl methacrylate. When the molecular weight of the alkyl methacrylate, such a PMMA, is between about 25,000 grams per mole and about 200,000 grams per mole, solvated highly crystalline PVDF, or highly crystalline copolymers comprising highly crystalline polyvinylidene fluoride, will mix uniformly without significantly retarding crystallization thus resulting in a coating composition with better hardness, solvent resistance and gloss retention then other known PVDF coatings. The high crystallinity of the vinylidene fluoride polymer and the uniform mixing achieved by use of polyalkyl methacrylate with a molecular weight from about 25,000 grams per mole to about 200,000 grams per mole provide the coating compositions with these favorable properties. Highly crystalline polymers comprising vinylidene fluoride, are a hompolymer of vinylidene fluoride or a copolymer of more than about 80% vinylidene fluoride and up to about 20% hexafluoropropylene with a crystallinity of from about 20% to about 70%.
All percentages and ratios set forth herein are by weight/weight unless specifically designated otherwise.
The present invention is a weather resistant coating composition comprising a polymer comprising vinylidene fluoride having a molecular weight of up to about 600,000 grams per mole and polyalkyl methacrylates having a molecular weight between about 25,000 grams per mole and about 200,000 grams per mole, preferably about 100,000 grams per mole to about 150,000 grams per mole, which coating compositions have unexpected combined superior properties, including gloss retention, adhesion and solvent resistance. More specifically, the vinylidene fluoride polymer is either a homopolymer of vinylidene fluoride or a copolymer of more than about 80% vinylidene fluoride and up to about 20% hexafluoropropylene and the polyalkyl methacrylate is polymethyl methacrylate or copolymers of more than about 65% methyl methacrylate and up to about 35% ethyl methacrylate or butyl methacrylate. The weather resistant coating compositions comprise from about 50% to about 90% polymer comprising vinylidene fluoride and from about 10% to about 50% polyalkyl methacrylate.
The weather coating composition is made by solvation process. Alkyl methacrylate polymer, such as PMMA, is dissolved in a latent solvent and then the polymer comprising vinylidene fluoride, such as PVDF, is mixed with the PMMA solution. Pigments and other fillers or additives may also be added to the mixture. The mixture is heated so that the solvent evaporates resulting in the coating composition. It is discovered that mixing of the polymers in this manner allows the polyvinylidene fluoride to form a highly crystalline blend after baking, thus resulting in a weather coating composition with highly crystalline polyvinylidene fluoride, such that the polyvinylidene fluoride, or polymer comprising polyvinylidene fluoride, has a crystallinity of about 20% to about 70%. The highly crystalline structure and the mixing that occurs when polyalkyl methacrylate having the particular molecular weight range is used results in a coating composition with unexpectedly superior properties, including hardness, adhesion and solvent resistance, in addition to gloss retention.
One of the purposes of the invention is obtaining a weather resistant coating composition with superior physical properties. Compositions in accordance with the invention exhibited greater solvent resistance and gloss retention. The coating compositions, for example, had about 100 percent gloss retention after 5,000 hours of QUV exposure. Whereas, coating compositions formulated with PVDF and a commercially available acrylic resin, had about 70% gloss retention after 5,000 hours. The excellent and unexpected gloss retention can be attributed, in part, to the molecular weight of the alkyl methacrylate being between about 25,000 grams per mole and about 200,000 grams per mole which results in more uniform mixing during the solvation process and a stronger composition.